1. Field of the Invention
This invention relates to rotary dynamoelectric machines, and particularly to disc-type variable reluctance motors.
2. Description of the Prior Art
Electromechanical energy conversion devices operate according to certain basic physical laws, one of which is known as the variable reluctance principle. According to that principle, in the operation of an electromechanical energy conversion device forces are established which tend to decrease the reluctance of the mechanical structure by moving its mechanical parts toward the configuration of minimum reluctance. This principle has been used in a variety of devices for producing mechanical force or torque, and for producing continuous rotation of a shaft.
Rotary dynamoelectric machines which utilize the variable reluctance principle are presently available in a wide variety of designs adapted for particular use as a motor or generator, for use with alternating or direct current systems, and for producing certain specific performance characteristics. The most common of these machines generally utilizes a rotor turning within an annular stator, the two components being separated by a concentric annular air gap. In such machines the primary electromagnetic field is directed radially with respect to the axis of rotation.
A distinctly different variable reluctance configuration employs mutually rotatable members constructed in the form of discs which are axially spaced from one another. At least one disc portion is axially spaced from another disc portion to define at least one radial air gap extending between adjacent pairs of these disc portions. At least one of these disc portions is stationary and constitutes a machine stator and at least one other disc portion is rotatable and constitutes a machine rotor. Each disc is composed of alternate magnetic segments and high resistivity non-magnetic segments. The primary electromagnetic field, produced by an annular solenoid which circumscribes the stator discs, is axially directed through the magnetic circuit formed by the interleaved rotor and stator discs. Such a machine has been described by P. French, U.S. Pat. No. 3,401,284, Sept. 10, 1968.
The rotary disc variable reluctance configuration is particularly useful in the design of AC synchronous reluctance motors where high power density ratios are required. To a first approximation, the power output of some conventional motors, such as AC induction motors and DC motors, is proportional to motor volume and output shaft speed. Changing the number of poles and the frequency of excitation has no significant effect on the power output of such motors as long as the shaft speed is fixed; that is to say the horsepower-to-weight ratio or the power density of the motor cannot be significantly altered by changing the characteristics of the input electrical power if the output mechanical shaft speed is constant. However, the power density of disc-type reluctance motors does respond to changes in the frequency of excitation. This characteristic can be used to advantage when variable speed drives are utilized since the frequency of excitation may be chosen at the convenience of the designer.
The disadvantage of prior art disc reluctance motors is that the input power factor, as defined by the ratio of watts input to VA input, is extremely low (typically 0.1 to 0.2). Also the ratio of peak torque to average torque is approximately 3.0 for the single phase disc reluctance machine. Further, in the single phase disc reluctance machine it is possible to stop the rotor in a position in which the rotor magnetic segments are completely aligned or are completely misaligned with the stator magnetic segments. In either case no starting torque will result.
It has been suggested in the prior art that the input power factor for the disc reluctance motor is improved by inducing an axially directed static magnetic field of arbitrary magnitude within the magnetic circuit formed by the interleaved rotor and stator discs. However, a static field structure which realizes optimum power factor with respect to maximum average torque has not been disclosed by prior art devices.
It is well known that a polyphase motor provides a better ratio of peak torque to average torque than does a single phase motor. Accordingly, a two-phase disc reluctance motor has been disclosed which provides relatively smooth torque output. It has also been suggested by W. Heintz in U.S. Pat. No. 3,700,944, Oct. 24, 1972, that the problem of insufficient starting torque due to complete alignment or complete misalignment of disc portions can be overcome by use of the multi-phase disc construction provided that the magnetic portions of either the rotor or stator discs are displaced an arbitrary amount with respect to corresponding magnetic portions of other rotors or stators. However, a rational rotor-stator alignment arrangement for a polyphase disc motor which achieves maximum available starting torque regardless of rotor position has not been disclosed by prior art devices.